Antenna designs for communication between a wirelessly powered implant to an external device outside the body

ABSTRACT

Methods and apparatus for wireless power transfer and communications are provided. In one embodiment, a wireless power transfer system comprises an external transmit resonator configured to transmit wireless power, an implantable receive resonator configured to receive the transmitted wireless power from the transmit resonator, and communications antenna in the implantable receive resonator configured to send communication information to the transmit resonator. The communications antenna can include a plurality of gaps positioned between adjacent conductive elements, the gaps being configured to prevent or reduce induction of current in the plurality of conductive elements when the antenna is exposed to a magnetic field.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Appln. No.62/053,663, titled “Antenna Designs for Communication Between aWirelessly Powered Implant to an External Device Outside the Body”,filed Sep. 22, 2014, which is incorporated herein by reference in itsentirety.

INCORPORATION BY REFERENCE

All publications and patent applications mentioned in this specificationare herein incorporated by reference to the same extent as if eachindividual publication or patent application was specifically andindividually indicated to be incorporated by reference.

FIELD

The field relates generally to resonant wireless power transfer systems,and more specifically to communication systems and methods forimplantable resonant wireless power transfer systems.

BACKGROUND

Many types of devices require transmitting energy between locations.Recent advances have accelerated the pace of innovation for wirelessenergy transmission (WET) without the use of cords. An example of asystem using wireless energy technology is a powered, implantablemedical device.

Many implantable medical devices require electrical systems to power theimplant. Typically, this is achieved using percutaneous wiring toconnect a power source to the implant. More recently, there has beeninterest in development of Transcutaneous Energy Transfer (TET) systems,e.g., through an oscillating magnetic field, for powering implantablemedical devices.

A TET system usually includes a number of components or systems. Aconventional TET system is implemented with a transmitting coil and areceiving coil for transmitting energy across the skin layer. The systemtypically includes a controller for driving the transmitting coil and/orcontrolling the implanted electronics.

Typically, implantable medical devices, such as implanted sensors,require very little power to operate. With such low power levels (on theorder of milliwatts), power transfer levels and efficiency can be lower.With higher power devices (e.g., on the order of watts and up to 15 W ormore), efficient transfer of wireless power is extremely important.Additionally, positions within the body are limited that can accommodatelarger implanted devices, some of which are deep below the skin surface.These implant locations require additional attention to position andorientation of both the transmit and receive coils, as well astechniques to improve and maximize transfer efficiency.

Previous TET systems for implantable medical devices required theimplanted receiver coil to be positioned just under the skin, andtypically include a mechanical feature to align the receive and transmitcoils and keep them together. By implanting these devices directly underthe skin, the size and power requirements of these implanted devices islimited if they are to be powered by a TET system. TET systems can bedesigned for operation even while power is not being received by thereceiver coil. In a typical configuration, solid-state electronics and abattery can power the implanted medical device when external power isinterrupted or not available. In this case, it may be beneficial toprovide a user interface or other electronic device to communicateinformation to the patient and/or caregiver regarding the implantedcomponents. For example, a user interface may include alarms to notifythe patient when the internal battery level is low.

Reliable communication between an implantable medical device, a userinterface, and an external transmitter can be a challenge because ofvarying conditions and distances between the components of the TETsystem.

Radio signals have limitations when used for communication betweenimplantable devices. Attenuation of radio signals by the human body isvery large and can disrupt communication signals. Even under optimalcircumstances, such as a shallow implant depth, a properly designedantenna, proper orientation of the implanted module, and a reliableradio link, attenuation can be on the order of 10 dB to 20 dB. Fordeeper implant depths, or if the implant rotates significantly from itsintended position, attenuation may grow to 100 dB or more. This can leadto an unreliable or totally interrupted radio link with a high lossrate.

In-band communication has been used in implanted systems and comprisesmodulation of a receiver load that can be sensed by a transmitter. Theproblem with in-band communication is that it requires additionalelectronics in the resonant circuit, which lowers the power transferefficiency and leads to additional heating of the receiver.Additionally, there is a fundamental design conflict between optimizinga resonant circuit to be power efficient and to transmit a meaningfulamount of information. The former requires coils with a high qualityfactor while the latter prefers lower quality factors.

It is therefore desirable to provide a system in which the implant cancommunicate effectively with the user interface in the absence of thetransmitter.

BRIEF DESCRIPTION OF THE DRAWINGS

The novel features of the invention are set forth with particularity inthe claims that follow. A better understanding of the features andadvantages of the present invention will be obtained by reference to thefollowing detailed description that sets forth illustrative embodiments,in which the principles of the invention are utilized, and theaccompanying drawings of which:

FIG. 1 illustrates a basic wireless power transfer system.

FIG. 2 illustrates the flux generated by a pair of coils.

FIGS. 3A-3B illustrate the effect of coil alignment on the couplingcoefficient.

FIG. 4 is one embodiment of a wireless power transfer system including acommunications antenna on an implantable TETS receiver.

FIG. 5 shows one embodiment of a communications antenna for use with awireless power transfer system.

FIG. 6 shows another embodiment of a communications antenna for use witha wireless power transfer system.

FIG. 7 shows yet another embodiment of a communications antenna for usewith a wireless power transfer system.

FIG. 8 shows a cross-sectional view of a communications antenna for usewith a wireless power transfer system.

FIG. 9 illustrates antenna match parameterized with increasing gapbetween conductive elements of a communications antenna.

FIG. 10 illustrates antenna match (reflection coefficient) parameterizedwith increasing gap.

SUMMARY OF THE DISCLOSURE

An antenna for use in a wireless power system is provided, comprising adifferential transmission line, a plurality of conductive elementscoupled to the differential transmission line and configured to radiateRF energy to transmit and receive radio information, and a plurality ofgaps positioned between adjacent conductive elements, the gaps beingconfigured to prevent or reduce induction of current in the plurality ofconductive elements when the antenna is exposed to a magnetic field.

In some embodiments, the plurality of conductive elements are arrangedso as to provide a first electrical path and a second electrical path.In other embodiments, the first electrical path comprises an interiortrace of the antenna, and the second electrical path comprises anexterior trace of the antenna. In another embodiment, the firstelectrical path is tuned to a first resonant frequency, and the secondelectrical path is tuned to a second resonant frequency.

A wireless receiver for use in a TET system is provided, comprising ahermetic internal housing, an energy source disposed in the internalhousing, a controller disposed in the housing, the controller configuredto control operation of the TET receiver, a low-frequency ferritehousing disposed around the internal housing, the ferrite housingconfigured to reduce the amount of magnetic flux that reaches theinternal housing, at least one wire coil wrapped around the ferritehousing, the at least one wire coil configured to receive wirelessenergy from an external power transmitter, an antenna body, a pluralityof conductive elements disposed on the antenna body, a differentialtransmission line electrically connecting the plurality of conductiveelements to the controller and configured to deliver RF energy to theplurality of conductive elements to transmit and receive radioinformation, and a plurality of gaps positioned between adjacentconductive elements, the gaps being configured to prevent or reduceinduction of current in the plurality of conductive elements when theantenna is exposed to a magnetic field.

In some embodiments, the plurality of conductive elements are arrangedso as to provide a first electrical path and a second electrical path.In other embodiments, the first electrical path comprises an interiortrace of the antenna, and the second electrical path comprises anexterior trace of the antenna. In another embodiment, the firstelectrical path is tuned to a first resonant frequency, and the secondelectrical path is tuned to a second resonant frequency.

In one embodiment, the receiver further comprises a high-frequencyferrite material positioned between the plurality of conductive elementsand the low-frequency ferrite material.

An implantable antenna assembly for communicating with an externaldevice is provided, comprising a housing formed of metal for enclosingelectrical elements, a low-frequency ferrite layer disposed on an outersurface of the housing, an antenna body disposed over the low-frequencyferrite layer, and an antenna metal disposed on the antenna body.

In some embodiments, the antenna metal is a resonant structure formed ofmetal strips.

In one embodiment, the metal strips are formed in a pattern having aregion of symmetry.

In one embodiment, the antenna further comprises a high-frequencyferrite layer disposed between the low-frequency ferrite layer and theantenna body.

In another embodiment, the antenna further comprises a conductive wellextending from within the housing to the antenna metal. In oneembodiment, the conductive well comprises the feedthrough pins formedwithin a ceramic.

In some embodiments, the antenna further comprises feedthrough pinsextending from within the housing to the antenna metal.

In one embodiment, the antenna further comprises a clearance layerseparating the high-frequency ferrite layer from the housing.

In some embodiments, the antenna assembly does not include a groundplane.

A communications antenna is provided, comprising a differentialtransmission line, a plurality of conductive elements coupled to thedifferential transmission line and configured to radiate RF energy totransmit and receive radio information, and a plurality of gapspositioned between adjacent conductive elements, the gaps beingconfigured to prevent or reduce induction of current in the plurality ofconductive elements when the antenna is exposed to a magnetic field.

DETAILED DESCRIPTION

In the description that follows, like components have been given thesame reference numerals, regardless of whether they are shown indifferent embodiments. To illustrate an embodiment(s) of the presentdisclosure in a clear and concise manner, the drawings may notnecessarily be to scale and certain features may be shown in somewhatschematic form. Features that are described and/or illustrated withrespect to one embodiment may be used in the same way or in a similarway in one or more other embodiments and/or in combination with orinstead of the features of the other embodiments.

Various aspects of the invention are similar to those described inInternational Patent Pub. No. WO2012045050; U.S. Pat. Nos. 8,140,168;7,865,245; 7,774,069; 7,711,433; 7,650,187; 7,571,007; 7,741,734;7,825,543; 6,591,139; 6,553,263; and 5,350,413; and U.S. Pub. Nos.2010/0308939; 2008/027293; and 2010/0102639, the entire contents ofwhich patents and applications are incorporated herein for all purposes.

Wireless Power Transmission System

Power may be transmitted wirelessly by magnetic induction. In variousembodiments, the transmitter and receiver are closely coupled.

In some cases “closely coupled” or “close coupling” refers to a systemthat requires the coils to be very near each other in order to operate.In some cases “loosely coupled” or “loose coupling” refers to a systemconfigured to operate when the coils have a significant spatial and/oraxial separation, and in some cases up to distance equal to or less thanthe diameter of the larger of the coils. In some cases, “looselycoupled” or “loose coupling” refers a system that is relativelyinsensitive to changes in physical separation and/or orientation of thereceiver and transmitter.

In some cases “loosely coupled” or “loose coupling” refers to a systemconfigured to operate when the coils have a significant spatial and/oraxial separation, and in some cases up to distance equal to or less thanthe diameter of the larger of the coils. In some cases, “looselycoupled” or “loose coupling” refers a system that is relativelyinsensitive to changes in physical separation and/or orientation of thereceiver and transmitter. In some cases, “loosely coupled” or “loosecoupling” refers a highly resonant system and/or a system usingstrongly-coupled magnetic resonators.

In various embodiments, the transmitter and receiver are non-resonantcoils. For example, a change in current in one coil induces a changingmagnetic field. The second coil within the magnetic field picks up themagnetic flux, which in turn induces a current in the second coil. Anexample of a closely coupled system with non-resonant coils is describedin International Pub. No. WO2000/074747, incorporated herein for allpurposes by reference. A conventional transformer is another example ofa closely coupled, non-resonant system. In various embodiments, thetransmitter and receiver are resonant coils. For example, one or both ofthe coils is connected to a tuning capacitor or other means forcontrolling the frequency in the respective coil. Examples of closelycoupled system with resonant coils are described in International Pub.Nos. WO2001/037926; WO2012/087807; WO2012/087811; WO2012/087816;WO2012/087819; WO2010/030378; and WO2012/056365, U.S. Pub. No.2003/0171792, and U.S. Pat. No. 5,350,413 (now abandoned), incorporatedherein for all purposes by reference.

For given coil sizes and separations, coupling a given amount of powerrequires generating the same magnetic field strength for eitherinductive or resonant systems. This requires the same number ofampere-turns in the coils. In inductive systems, all the ampere-turnspass through the MOSFETs and generate power losses in their on-stateresistance. In resonant systems, only the exciter ampere-turns passthrough the MOSFETs, while the resonator ampere-turns do not. As aconsequence, resonant systems will always have lower losses and higherefficiencies than inductive systems of the same dimensions and powerthrough-put.

In various embodiments, the transmitter and receiver are non-resonantcoils. For example, a change in current in one coil induces a changingmagnetic field. The second coil within the magnetic field picks up themagnetic flux, which in turn induces a current in the second coil. Anexample of a closely coupled system with non-resonant coils is describedin International Pub. No. WO2000/074747, incorporated herein for allpurposes by reference. A conventional transformer is another example ofa closely coupled, non-resonant system. In various embodiments, thetransmitter and receiver are resonant coils. For example, one or both ofthe coils is connected to a tuning capacitor or other means forcontrolling the frequency in the respective coil. An example of closelycoupled system with resonant coils is described in International Pub.Nos. WO2001/037926; WO2012/087807; WO2012/087811; WO2012/087816;WO2012/087819; WO2010/030378; and WO2012/056365, and U.S. Pub. No.2003/0171792, incorporated herein for all purposes by reference.

In various embodiments, the transmitter and receiver are looselycoupled. For example, the transmitter can resonate to propagate magneticflux that is picked up by the receiver at relatively great distances. Insome cases energy can be transmitted over several meters. In a looselycoupled system power transfer may not necessarily depend on a criticaldistance. Rather, the system may be able to accommodate changes to thecoupling coefficient between the transmitter and receiver. An example ofa loosely coupled system is described in International Pub. No.WO2012/045050, incorporated herein for all purposes by reference.

Power may be transmitted wirelessly by radiating energy. In variousembodiments, the system comprises antennas. The antennas may be resonantor non-resonant. For example, non-resonant antennas may radiateelectromagnetic waves to create a field. The field can be near field orfar field. The field can be directional. Generally far field has greaterrange but a lower power transfer rate. An example of such a system forradiating energy with resonators is described in International Pub. No.WO2010/089354, incorporated herein for all purposes by reference. Anexample of such a non-resonant system is described in International Pub.No. WO2009/018271, incorporated herein for all purposes by reference.Instead of antenna, the system may comprise a high energy light sourcesuch as a laser. The system can be configured so photons carryelectromagnetic energy in a spatially restricted, direct, coherent pathfrom a transmission point to a receiving point. An example of such asystem is described in International Pub. No. WO2010/089354,incorporated herein for all purposes by reference.

Power may also be transmitted by taking advantage of the material ormedium through which the energy passes. For example, volume conductioninvolves transmitting electrical energy through tissue between atransmitting point and a receiving point. An example of such a system isdescribed in International Pub. No. WO2008/066941, incorporated hereinfor all purposes by reference.

Power may also be transferred using a capacitor charging technique. Thesystem can be resonant or non-resonant. Exemplars of capacitor chargingfor wireless energy transfer are described in International Pub. No.WO2012/056365, incorporated herein for all purposes by reference.

The system in accordance with various aspects of the invention will nowbe described in connection with a system for wireless energy transfer bymagnetic induction. The exemplary system utilizes resonant powertransfer. The system works by transmitting power between the twoinductively coupled coils. In contrast to a transformer, however, theexemplary coils are not coupled together closely. A transformergenerally requires the coils to be aligned and positioned directlyadjacent each other. The exemplary system accommodates looser couplingof the coils.

While described in terms of one receiver coil and one transmitter coil,one will appreciate from the description herein that the system may usetwo or more receiver coils and two or more transmitter coils. Forexample, the transmitter may be configured with two coils—a first coilto resonate flux and a second coil to excite the first coil. One willfurther appreciate from the description herein that usage of “resonator”and “coil” may be used somewhat interchangeably. In various respects,“resonator” refers to a coil and a capacitor connected together.

In accordance with various embodiments of this disclosure, the systemcomprises one or more transmitters configured to transmit powerwirelessly to one or more receivers. In various embodiments, the systemincludes a transmitter and more than one receiver in a multiplexedarrangement. A frequency generator may be electrically coupled to thetransmitter to drive the transmitter to transmit power at a particularfrequency or range of frequencies. The frequency generator can include avoltage controlled oscillator and one or more switchable arrays ofcapacitors, a voltage controlled oscillator and one or more varactors, aphase-locked-loop, a direct digital synthesizer, or combinationsthereof. The transmitter can be configured to transmit power at multiplefrequencies simultaneously. The frequency generator can include two ormore phase-locked-loops electrically coupled to a common referenceoscillator, two or more independent voltage controlled oscillators, orcombinations thereof. The transmitter can be arranged to simultaneouslydelivery power to multiple receivers at a common frequency.

In various embodiments, the transmitter is configured to transmit a lowpower signal at a particular frequency. The transmitter may transmit thelow power signal for a particular time and/or interval. In variousembodiments, the transmitter is configured to transmit a high powersignal wirelessly at a particular frequency. The transmitter maytransmit the high power signal for a particular time and/or interval.

In various embodiments, the receiver includes a frequency selectionmechanism electrically coupled to the receiver coil and arranged toallow the resonator to change a frequency or a range of frequencies thatthe receiver can receive. The frequency selection mechanism can includea switchable array of discrete capacitors, a variable capacitance, oneor more inductors electrically coupled to the receiving antenna,additional turns of a coil of the receiving antenna, or combinationsthereof.

In general, most of the flux from the transmitter coil does not reachthe receiver coil. The amount of flux generated by the transmitter coilthat reaches the receiver coil is described by “k” and referred to asthe “coupling coefficient.”

In various embodiments, the system is configured to maintain a value ofk in the range of between about 0.2 to about 0.01. In variousembodiments, the system is configured to maintain a value of k of atleast 0.01, at least 0.02, at least 0.03, at least 0.04, or at least0.05.

In various embodiments, the coils are physically separated. In variousembodiments, the separation is greater than a thickness of the receivercoil. In various embodiments, the separation distance is equal to orless than the diameter of the larger of the receiver and transmittercoil.

Because most of the flux does not reach the receiver, the transmittercoil must generate a much larger field than what is coupled to thereceiver. In various embodiments, this is accomplished by configuringthe transmitter with a large number of amp-turns in the coil.

Since only the flux coupled to the receiver gets coupled to a real load,most of the energy in the field is reactive. The current in the coil canbe sustained with a capacitor connected to the coil to create aresonator. The power source thus only needs to supply the energyabsorbed by the receiver. The resonant capacitor maintains the excessflux that is not coupled to the receiver.

In various embodiments, the impedance of the receiver is matched to thetransmitter. This allows efficient transfer of energy out of thereceiver. In this case the receiver coil may not need to have a resonantcapacitor.

Turning now to FIG. 1, a simplified circuit for wireless energytransmission is shown. The exemplary system shows a series connection,but the system can be connected as either series or parallel on eitherthe transmitter or receiver side.

The exemplary transmitter includes a coil Lx connected to a power sourceVs by a capacitor Cx. The exemplary receiver includes a coil Lyconnected to a load by a capacitor Cy. Capacitor Cx may be configured tomake Lx resonate at a desired frequency. Capacitance Cx of thetransmitter coil may be defined by its geometry. Inductors Lx and Ly areconnected by coupling coefficient k. Mxy is the mutual inductancebetween the two coils. The mutual inductance, Mxy, is related tocoupling coefficient, k.

Mxy=k√{square root over (Lx·Ly)}

In the exemplary system the power source Vs is in series with thetransmitter coil Lx so it may have to carry all the reactive current.This puts a larger burden on the current rating of the power source andany resistance in the source will add to losses.

The exemplary system includes a receiver configured to receive energywirelessly transmitted by the transmitter. The exemplary receiver isconnected to a load. The receiver and load may be connected electricallywith a controllable switch.

In various embodiments, the receiver includes a circuit elementconfigured to be connected or disconnected from the receiver coil by anelectronically controllable switch. The electrical coupling can includeboth a serial and parallel arrangement. The circuit element can includea resistor, capacitor, inductor, lengths of an antenna structure, orcombinations thereof. The system can be configured such that power istransmitted by the transmitter and can be received by the receiver inpredetermined time increments.

In various embodiments, the transmitter coil and/or the receiver coil isa substantially two-dimensional structure. In various embodiments, thetransmitter coil may be coupled to a transmitter impedance-matchingstructure. Similarly, the receiver coil may be coupled to a receiverimpedance-matching structure. Examples of suitable impedance-matchingstructures include, but are not limited to, a coil, a loop, atransformer, and/or any impedance-matching network. Animpedance-matching network may include inductors or capacitorsconfigured to connect a signal source to the resonator structure.

In various embodiments, the transmitter is controlled by a controller(not shown) and driving circuit. The controller and/or driving circuitmay include a directional coupler, a signal generator, and/or anamplifier. The controller may be configured to adjust the transmitterfrequency or amplifier gain to compensate for changes to the couplingbetween the receiver and transmitter.

In various embodiments, the transmitter coil is connected to animpedance-matched coil loop. The loop is connected to a power source andis configured to excite the transmitter coil. The first coil loop mayhave finite output impedance. A signal generator output may be amplifiedand fed to the transmitter coil. In use power is transferredmagnetically between the first coil loop and the main transmitter coil,which in turns transmits flux to the receiver. Energy received by thereceiver coil is delivered by Ohmic connection to the load.

One of the challenges to a practical circuit is how to get energy in andout of the resonators. Simply putting the power source and load inseries or parallel with the resonators is difficult because of thevoltage and current required. In various embodiments, the system isconfigured to achieve an approximate energy balance by analyzing thesystem characteristics, estimating voltages and currents involved, andcontrolling circuit elements to deliver the power needed by thereceiver.

In an exemplary embodiment, the system load power, P_(L), is assumed tobe 15 Watts and the operating frequency of the system, f, is 250 kHz.Then, for each cycle the load removes a certain amount of energy fromthe resonance:

$e_{L} = {\frac{P_{L}}{f} = {60\mspace{14mu} \mu \; J\mspace{14mu} {Energy}\mspace{14mu} {the}\mspace{14mu} {load}\mspace{14mu} {removes}\mspace{14mu} {in}\mspace{14mu} {one}\mspace{14mu} {cycle}}}$

It has been found that the energy in the receiver resonance is typicallyseveral times larger than the energy removed by the load for operative,implantable medical devices. In various embodiments, the system assumesa ratio 7:1 for energy at the receiver versus the load removed. Underthis assumption, the instantaneous energy in the exemplary receiverresonance is 420 μJ.

The exemplary circuit was analyzed and the self inductance of thereceiver coil was found to be 60 uH. From the energy and the inductance,the voltage and current in the resonator could be calculated.

$e_{y} = {\frac{1}{2}{Li}^{2}}$$i_{y} = {\sqrt{\frac{2e_{y}}{L}} = {3.74A\mspace{14mu} {peak}}}$v_(y) = ω L_(y)i_(y) = 352V  peak

The voltage and current can be traded off against each other. Theinductor may couple the same amount of flux regardless of the number ofturns. The Amp-turns of the coil needs to stay the same in this example,so more turns means the current is reduced. The coil voltage, however,will need to increase. Likewise, the voltage can be reduced at theexpense of a higher current. The transmitter coil needs to have muchmore flux. The transmitter flux is related to the receiver flux by thecoupling coefficient. Accordingly, the energy in the field from thetransmitter coil is scaled by k.

$e_{x} = \frac{e_{y}}{k}$

Given that k is 0.05:

$e_{x} = {\frac{420\mspace{14mu} \mu \; J}{0.05} = {8.4{mJ}}}$

For the same circuit the self inductance of the transmitter coil was 146uH as mentioned above. This results in:

$i_{x} = {\sqrt{\frac{2e_{x}}{L}} = {10.7A\mspace{14mu} {peak}}}$v_(x) = ω L_(x)i_(x) = 2460V  peak

One can appreciate from this example, the competing factors and how tobalance voltage, current, and inductance to suit the circumstance andachieve the desired outcome. Like the receiver, the voltage and currentcan be traded off against each other. In this example, the voltages andcurrents in the system are relatively high. One can adjust the tuning tolower the voltage and/or current at the receiver if the load is lower.

Estimation of Coupling Coefficient and Mutual Inductance

As explained above, the coupling coefficient, k, may be useful for anumber of reasons. In one example, the coupling coefficient can be usedto understand the arrangement of the coils relative to each other sotuning adjustments can be made to ensure adequate performance. If thereceiver coil moves away from the transmitter coil, the mutualinductance will decrease, and ceteris paribus, less power will betransferred. In various embodiments, the system is configured to maketuning adjustments to compensate for the drop in coupling efficiency.

The exemplary system described above often has imperfect information.For various reasons as would be understood by one of skill in the art,the system does not collect data for all parameters. Moreover, becauseof the physical gap between coils and without an external means ofcommunications between the two resonators, the transmitter may haveinformation that the receiver does not have and vice versa. Theselimitations make it difficult to directly measure and derive thecoupling coefficient, k, in real time.

Described below are several principles for estimating the couplingcoefficient, k, for two coils of a given geometry. The approaches maymake use of techniques such as Biot-Savart calculations or finiteelement methods. Certain assumptions and generalizations, based on howthe coils interact in specific orientations, are made for the sake ofsimplicity of understanding. From an electric circuit point of view, allthe physical geometry permutations can generally lead to the couplingcoefficient.

If two coils are arranged so they are in the same plane, with one coilcircumscribing the other, then the coupling coefficient can be estimatedto be roughly proportional to the ratio of the area of the two coils.This assumes the flux generated by coil 1 is roughly uniform over thearea it encloses as shown in FIG. 2.

If the coils are out of alignment such that the coils are at a relativeangle, the coupling coefficient will decrease. The amount of thedecrease is estimated to be about equal to the cosine of the angle asshown in FIG. 3A. If the coils are orthogonal to each other such thattheta (0) is 90 degrees, the flux will not be received by the receiverand the coupling coefficient will be zero.

If the coils are arraigned such that half the flux from one coil is inone direction and the other half is in the other direction, the fluxcancels out and the coupling coefficient is zero, as shown in FIG. 3B.

A final principle relies on symmetry of the coils. The couplingcoefficient and mutual inductance from one coil to the other is assumedto be the same regardless of which coil is being energized.

M _(xy) =M _(yx)

FIG. 4 illustrates a wireless power transfer system comprising animplantable TETS receiver unit 400 implanted in an abdomen of a humanpatient. The receiver unit 400 can be coupled to a device load 402, suchas an implantable medical device, e.g., an implantable LVAD or heartpump. The exemplary receiver unit 400 can include a receiver resonatorcoil and electronics configured to receive wireless energy from anexternal transmitter 401, which can include a power supply such as apulse generator connected to a transmitter resonator coil. In oneembodiment, the electronics and coils are implanted separately andconnected by an implanted cable. In some embodiments, externalcontroller 404 can be configured to communicate with the TETS receiver400 and can be worn by the patient, such as on the patient's wrist. Inother embodiments, the external controller can be an electroniccomputing device such as a personal computer, a tablet, smartphone, orlaptop computer.

In one embodiment, the receiver unit 400 further includes acommunications antenna 406 disposed along an outer periphery, theantenna being configured to send and receive communications data to andfrom other electronic devices inside and outside of the body. In oneembodiment, the receiver unit further includes an internal rechargeablepower source. In various embodiments, the receiver unit 400 of the TETsystem is configured as a single implanted device including the receivecoil, antenna, power source, and associated circuitry. The receiver unitis configured so the implantable medical device can be plugged directlyinto the unit. The single housing configuration makes implantationeasier and faster. Additionally, since there are less implants, andconsequently less tunneling in the body and percutaneous defect sites,adverse event risks like bleeding and infection are reduced. One ofskill will appreciate from the description herein that various internalcomponents of the system can be bundled together or implantedseparately. For example, the internal rechargeable power source can beimplanted separately from the receiver unit and connected by a powercable. The antenna assembly, power source, and receive coil can all beconfigured in separate hermetically sealed housings. International Pub.No. WO2007/053881A1, U.S. Pub. No. 2014/0005466, and U.S. Pat. No.8,562,508, the entire contents of which are incorporated herein for allpurposes by reference, disclose several exemplary configurations.

FIG. 5 illustrates a top-down view of one embodiment of a communicationsantenna 506 configured to send and receive communications data from animplanted wireless powered device to external devices. Thecommunications antenna 506 can be disposed on or in the receiver unitillustrated in FIG. 4. The communications antenna 506 can comprise atleast one feedline 507 that connects the communications antenna to theradio transmitter and/or receiver, and a plurality of conductiveelements 508 a-h coupled to the feedline. While conductive elements 508a and 508 d are physically connected to the feedline (e.g., with a wireor conductive connection), adjacent conductive elements in thecommunications antenna are not physically connected, but instead areseparated by gaps 510 a-p. The gaps can be, for example, air gapsbetween adjacent conductive elements. More specifically, adjacentconductive elements are not electrically connected to each other, suchas with a conductive wire or trace, but instead are separated from eachother by the gaps. In the illustrated embodiment, gap 510 a separatesconductive elements 508 a and 508 b, gap 510 b separates conductiveelements 508 b and 508 c, gap 510 c separates conductive elements 508 cand 508 d, gap 510 d separates conductive elements 508 d and 508 e, gap510 e separates conductive elements 508 e and 508 f, gap 510 f separatesconductive elements 508 f and 508 a. Similarly, gap 510 g separatesconductive elements 508 a and 508 g, gap 510 h separates conductiveelements 508 b and 508 g, gap 510 i separates conductive elements 508 cand 508 g, gap 510 j separates conductive elements 508 d and 508 g, gap510 k separates conductive elements 508 d and 508 h, gap 510 l separatesconductive elements 508 e and 508 h, gap 510 m separates conductiveelements 508 f and 508 h, and gap 510 n separates conductive elements508 a and 508 h. Finally, gaps 510 o and 510 p separate conductiveelements 508 g and 508 h on the perimeter of the communications antenna.

As is known in the art, currents can be induced in a loop of conductoror metal when exposed to a magnetic field. Therefore, a looped conductorplaced in the TETS field of a magnetically coupled wireless powertransfer system can result in potentially large currents being inducedin the looped conductor by the TETS field. In this example, an antennacan be saturated by the wireless power transfer. To solve this problem,the communications antennas described herein can include gaps betweenadjacent conductive elements to minimize low-frequency coupling in theantenna. These antennas can be designed without any closed paths thatcan be traced to induce currents in the presence of a TETS field. Insome embodiments, a wireless power transfer system (such as the systemdescribed in FIG. 4) can have a low operating frequency, such as anoperating frequency of 250 kHz. By comparison, the communication antennafrequency of the exemplary system is sufficiently higher (e.g., over 100MHz) such that the TET system does not interfere with the datacommunication system. Due to the induced power loss in the antenna,especially relative to the loop conductor described above, traditionalpatch antennas and any antenna with a ground plane would be unsuitablefor this application in the presences of a TETS field. The gaps in thecommunications antenna, therefore, can be configured to prevent inducedcurrents in the antenna during wireless power transfer at this operatingfrequency. In other words, the exemplary structure includes gapsdesigned to reduce or minimize low-frequency coupling. The location andspacing of these gaps can be modified and optimized to control theoperation and resonant frequencies of the communications antennarelative to the TET system frequency. For example, the width andposition of the traces can be changed (e.g. midway through signal flow)to improve performance and coupling at a first frequency range whileminimizing coupling at a second frequency range.

In the embodiment of FIG. 5, the conductive elements can comprise aplurality of electrical paths, including an interior trace and anexterior trace. In some embodiments, these electrical paths can be tunedto the same resonant frequencies. In some embodiments, these electricalpaths can be tuned to different resonant frequencies. The interior tracecan comprise the electrical path from feedline 507 to conductive element508 a, to conductive element 508 b, to conductive element 508 c, toconductive element 508 d, back to feedline 507. The interior trace canalso comprise the electrical path from feedline 507 to conductiveelement 508 d, to conductive element 508 e, to conductive element 508 f,to conductive element 508 a, back to feedline 507. Still referring toFIG. 5, the exterior trace can comprise the electrical path fromfeedline 507 to conductive element 508 a, to conductive element 508 galong the perimeter of the antenna, to conductive element 508 d, back tofeedline 507. The exterior trace can also comprise the electrical pathfrom feedline 507 to conductive element 508 d, to conductive element 508h along the perimeter of the antenna, to conductive element 508 a, backto feedline 507. The choice of interior vs. exterior path, or which pathcomposed of a plurality of conductive elements, depends on the degree towhich the path is tuned to resonance at the frequency in question.

As described above, the exemplary communications antenna 506 cancomprise two paths, an exterior trace and an interior trace. Each ofthese paths can be tuned to two different resonant frequencies. If theantenna is driven at the lower of the two resonance frequencies, the RFenergy tends to go around the outer path or exterior trace of theantenna. If the antenna is driven at the higher of the two resonancefrequencies, the RF energy tends to go around the internal trace of theantenna. When the antenna is driven in the frequencies between tworesonance frequencies, the RF energy radiates though both the interiorand exterior traces of the antenna. The length of the interior andexterior traces is determined by the wavelength of the radio waves usedin the antenna. Since the interior trace can be designed to radiate at adifferent resonant frequency than the exterior trace, the total lengthof each trace can be designed based on the needs of the antenna. In someembodiments, both the interior and exterior traces can be configured asquarter-wave antennas, half-wave antennas, 5/8 wave antennas, full-waveantennas, or any other appropriate wavelength. In one specificembodiment, the interior path can comprise a half-wave antenna and theexterior path can comprise a 3/2 wave antenna. In some embodiments, theelectrical length of the interior trace can be different than theelectrical length of the exterior trace. For example, the interior tracecan comprise a half-wave antenna, and the exterior trace can comprise afull-wave antenna, or other appropriate combinations.

As shown in FIG. 5, communications antenna 506 can be mirrored along oneor more lines of symmetry. Symmetrical designs tend to createsymmetrical radiation patterns. In this embodiment, the top/bottomsymmetry is strongly suggested by the symmetry of the feed points 607.In the illustrated embodiment, the top portion of the antenna(conductive elements 508 h, 508 f, 508 a, 508 b, and 508 g) can mirrorthe bottom portion of the antenna (conductive elements 508 h, 508 e, 508d, 508 c, and 508 g). Similarly, in FIG. 5, the left portion of theantenna (conductive elements 508 h, 508 f, 508 e, 508 a, and 508 d) canmirror the right portion of the antenna (conductive elements 508 g, 508b, 508 c, 508 d, and 508 a).

FIG. 6 illustrates another embodiment of a communications antenna 606.The antenna 606 is similar to the antenna of FIG. 5, but with a slightlydifferent arrangement of conductive elements. In FIG. 6, antenna 606includes feedline 607, conductive elements 608 a-f, and gaps 510 a-j.Similar to the embodiment of FIG. 5, antenna 606 can also include aninterior trace and an exterior trace. The electrical path along theinterior trace can comprise, for example, a path from the feedline 607to conductive elements 608 a, 608 b, 608 c, back to feedline 607 (orreversed), or alternatively, a path from the feedline 607 to conductiveelements 608 c, 608 d, 608 a, back to feedline 607 (or reversed).Similarly, the electrical path along the exterior trace can comprise,for example, a path from the feedline 607 to conductive elements 608 a,608 e, 608 c, back to feedline 607 (or reversed), or alternatively, apath from the feedline 607 to conductive elements 608 c, 608 f, 608 a,back to feedline 607 (or reversed). As described above, the interior andexterior traces of the antenna can be tuned to different resonantfrequencies. Also shown in FIG. 6, the conductive elements can have alooping design wherein the individual conductive elements are bent orturned so as to arrive at the desired antenna length. It should be notedthat even when the conductive elements are bent into a loop-like shape,the loop is not closed and includes a small gap so as to prevent theinduction of low-frequency current in the elements. However,high-frequency (antenna frequency) currents must flow or the antennadoesn't radiate.

FIG. 7 illustrates one embodiment in which one or more of the conductiveelements 708 a-d can include a meander or meandering design, in whichthe conductive element zig-zags in different directions to achieve thedesired electrical length.

FIG. 8 is a cross-sectional view of a communications antenna 806 mountedon a receiver unit 800 (such as the receiver unit of FIG. 4). Theantenna 806 can comprise a plurality of conductive elements 808 whichmake up the resonators of the antenna. The exemplary antenna does notcomprise a ground plane. The conductive elements can comprise anyconductive material, such as aluminum, copper, silver, gold, or thelike. The conductive elements can rest on an antenna body 812. Theantenna body can comprise an insulator, such as ceramics, glasses,certain polymers, ceramic powders, glass fibers bonded with polymers,mineral or mineral oxide crystals (sapphire, diamond), or the like. Insome embodiments, the antenna body can have a high dielectric constantconfigured to cause the antenna to appear electrically larger.

In the exemplary embodiment, the receiver unit 800 is a fully integratedantenna. A resonant structure is integrated into the antenna structurewithout the use of a ground plane. The exemplary receiver unit 800includes a housing or can 814 comprising a biocompatible material suchas titanium. The housing 814 can be hermetically sealed to contain thereceiving and power management electronics of the receiver unit. In someembodiments, the housing 814 can be surrounded with a low-frequencyferrite material 816, which can be held in place over the housing 814with an adhesive layer 818. The low-frequency ferrite material can beconfigured to prevent the magnetic fields of the wireless power systemfrom exciting circulating currents in the housing. For example, thelow-frequency ferrite material can be configured to block or diminishmagnetic fields in the operating frequency of the wireless power system(e.g., frequencies in the range of 250 kHz).

A high-frequency ferrite material 820 can be disposed between theantenna body 812 and the low-frequency ferrite material 816. In someembodiments, the low-frequency ferrite material 816 can be extremelylossy at the frequencies used by the communications antenna. Thehigh-frequency ferrite material can serve as a spacer to separate theantenna from the low-frequency ferrite material of the receiver unit,and can also serve to reflect the downward facing energy from thecommunications antenna back upwards towards the intended communicationstarget. This high-frequency ferrite material can prevent the downwardfacing energy from being dissipated by the low-frequency ferritematerial. In some embodiments, the high-frequency ferrite material canhave a low permeability and permittivity. The entire assembly, includingthe antenna 806 and the receiver unit 800 can be encapsulated with anencapsulation layer 730, which can comprise a plastic, ceramic, oradhesive material.

A feedthrough subassembly comprising a conductive well 822, feedthroughceramics 824, and feedthrough pins 826, is designed to behave ascontrolled impedance transmission line between the electronics of thereceiver unit and the communications antenna. A clearance layer 828 canbe included during manufacturing for mechanical tolerance, and canprovide an empty space that is backfilled with an adhesive to ensurethat all the components fit together properly.

In FIG. 8, the antenna and housing design can include an inductorconfigured to shunt any TETS energy picked up by the antenna away fromthe electronics inside the can. In some embodiments, there may be somefinite pickup of the TETS signal by the antenna even with the gapsseparating the conductive elements. The inductor value should be aslarge as possible to shunt away any energy picked up. In someembodiments, the self-resonant frequency of the inductor is 25% higherthan the RF frequency of the antenna. In some embodiments, theself-resonant frequency of the inductor is at least 5% higher, at least10% higher, at least 15% higher, or at least 20% higher than the RFfrequency of the antenna. In some embodiments, the self-resonantfrequency of the inductor is at least 50% higher than the RF frequencyof the antenna. In some embodiments, the self-resonant frequency of theinductor is an order of magnitude higher than the RF frequency of theantenna.

FIG. 9 illustrates antenna match (return loss) parameterized withincreasing gap between the conductive elements of the antenna. Theresonance frequency can be adjusted by varying the meander portion ofthe pattern, but typically multiple parameters should be adjusted inconcert to tune performance. In one embodiment, a lower value of returnloss is desired in the antenna design.

FIG. 10 illustrates antenna match (reflection coefficient) parameterizedwith increasing gap. A value close to the center (0) is desired in theantenna design. This is the same data as shown in FIG. 9, but plotted ascomplex reflection.

Although various illustrative embodiments are described above, any of anumber of changes may be made to various embodiments without departingfrom the scope of the invention as described by the claims. For example,the order in which various described method steps are performed mayoften be changed in alternative embodiments, and in other alternativeembodiments one or more method steps may be skipped altogether. Optionalfeatures of various device and system embodiments may be included insome embodiments and not in others. Therefore, the foregoing descriptionis provided primarily for exemplary purposes and should not beinterpreted to limit the scope of the invention as it is set forth inthe claims. Although described in some respects as a medical system, onewill appreciate from the description herein that the principles canapply equally to other types of systems including, but not limited to,consumer electronics, automotive, phones and personal communicationdevices, gaming devices, and computers and peripherals.

The examples and illustrations included herein show, by way ofillustration and not of limitation, specific embodiments in which thesubject matter may be practiced. As mentioned, other embodiments may beutilized and derived there from, such that structural and logicalsubstitutions and changes may be made without departing from the scopeof this disclosure. Such embodiments of the inventive subject matter maybe referred to herein individually or collectively by the term“invention” merely for convenience and without intending to voluntarilylimit the scope of this application to any single invention or inventiveconcept, if more than one is, in fact, disclosed. Thus, althoughspecific embodiments have been illustrated and described herein, anyarrangement calculated to achieve the same purpose may be substitutedfor the specific embodiments shown. This disclosure is intended to coverany and all adaptations or variations of various embodiments.Combinations of the above embodiments, and other embodiments notspecifically described herein, will be apparent to those of skill in theart upon reviewing the above description.

1-9. (canceled)
 10. An implantable antenna assembly for communicatingwith an external device, comprising: a hermetically sealed housingformed of metal for enclosing receiver electronics; a low-frequencyferrite layer disposed on and surrounding an outer surface of thehermetically sealed housing, the low-frequency ferrite layer configuredto reduce the amount of magnetic flux that reaches the hermeticallysealed housing; an antenna body disposed over the low-frequency ferritelayer; and an antenna metal disposed on the antenna body.
 11. Theantenna assembly of claim 10, wherein the antenna metal is a resonantstructure formed of metal strips.
 12. The antenna assembly of claim 11,wherein the metal strips are formed in a pattern having a region ofsymmetry.
 13. The antenna assembly of claim 10, further comprising ahigh-frequency ferrite layer disposed between the low-frequency ferritelayer and the antenna body.
 14. The antenna assembly of claim 10,further comprising a conductive well extending from within thehermetically sealed housing to the antenna metal.
 15. The antennaassembly of claim 10, further comprising feedthrough pins extending fromwithin the hermetically sealed housing to the antenna metal.
 16. Theantenna assembly of claim 15, wherein the conductive well comprises thefeedthrough pins formed within a ceramic.
 17. The antenna assembly ofclaim 10, further comprising a clearance layer separating thehigh-frequency ferrite layer from the hermetically sealed housing. 18.The antenna assembly of claim 10, wherein the antenna assembly does notinclude a ground plane.
 19. (canceled)
 20. The antenna assembly of claim10, further comprising: a differential transmission line electricallyconnecting a plurality of conductive elements of the antenna metal tothe receiver electronics and configured to deliver RF energy to theplurality of conductive elements to transmit and receive radioinformation; and a plurality of gaps positioned between adjacentconductive elements, the gaps configured to prevent or reduce inductionof current in the plurality of conductive elements when the antenna isexposed to a magnetic field.
 21. The antenna assembly of claim 20,wherein the plurality of conductive elements define a first electricalpath and a second electrical path.
 22. The antenna assembly of claim 21,wherein the first electrical path comprises an interior trace, and thesecond electrical path comprises an exterior trace.
 23. The antennaassembly of claim 21, wherein the first electrical path is tuned to afirst resonant frequency, and the second electrical path is tuned to asecond resonant frequency.
 24. The antenna assembly of claim 20, furthercomprising a high-frequency ferrite material positioned between theplurality of conductive elements and the low-frequency ferrite material.25. An implantable antenna assembly for communicating with an externaldevice, comprising: a hermetically sealed housing formed of metal forenclosing receiver electronics; a low-frequency ferrite layer disposedon and surrounding an outer surface of the hermetically sealed housing,the low-frequency ferrite layer configured to reduce the amount ofmagnetic flux that reaches the hermetically sealed housing; an antennabody disposed over the low-frequency ferrite layer; and a plurality ofconductive elements disposed on the antenna body, wherein a plurality ofgaps are positioned between adjacent conductive elements, the gapsconfigured to prevent or reduce induction of current in the plurality ofconductive elements when the antenna is exposed to a magnetic field. 26.The antenna assembly of claim 25, wherein the plurality of conductiveelements define a first electrical path and a second electrical path.27. The antenna assembly of claim 26, wherein the first electrical pathcomprises an interior trace, and the second electrical path comprises anexterior trace.
 28. The antenna assembly of claim 26, wherein the firstelectrical path is tuned to a first resonant frequency, and the secondelectrical path is tuned to a second resonant frequency.
 29. The antennaassembly of claim 25, further comprising a high-frequency ferritematerial positioned between the plurality of conductive elements and thelow-frequency ferrite material.